MRAMs (Magnetoresistive Random Access Memories) are memories using magnetization reversals. Spin-injection MRAMs using spin torque transfer switching writing excel in speed, integration properties, and durability, and are expected as general-purpose nonvolatile random access memories.
MTJ (Magnetic Tunnel Junction) elements are used as storage elements of spin-injection MRAMs. An MTJ element has a three-layer structure that includes: a storage layer including a magnetic layer having a magnetization direction that is switched by a memory write operation; a reference layer including a magnetic layer having a magnetization direction that is fixed in one direction; and a tunnel barrier layer that is interposed between the storage layer and the reference layer, and forms a tunnel barrier. Depending on whether the magnetization directions of the storage layer and the reference layer are parallel or antiparallel, the electrical resistance becomes a low resistance or a high resistance when a current is applied in a direction perpendicular to the film plane of the MTJ element. By using the resistance difference between the parallel state and the antiparallel state, data can be read from the MTJ element serving as a storage element.
Spin torque transfer switching writing is performed by adjusting the magnetization direction of the storage layer through flowing current in the direction perpendicular to the film plane of the MTJ element. To switch the magnetization from the antiparallel state to the parallel state, a current is applied in such a direction that electrons flow from the reference layer toward the storage layer. The current direction is the direction from the storage layer toward the reference layer. Through flowing the current, a spin torque to reverse a magnetization direction to a direction parallel to the magnetization direction of the reference layer is applied to the magnetization of the storage layer. By flowing a current equal to or higher than a threshold value, the magnetization of the storage layer can be changed. To switch the magnetization from the parallel state to the antiparallel state, a current is flowed in such a direction that electrons flow from the storage layer toward the reference layer. A spin torque acts on the magnetization of the storage layer, so that the magnetization is switched to the direction antiparallel to the magnetization direction of the reference layer. Data can be rewritten by changing current flow directions in the above manner.
There have been MTJ elements using perpendicular magnetization films each having a magnetization direction perpendicular to the film plane, and MTJ elements using in-plane magnetization films each having a magnetization direction parallel to the film plane. In an MTJ element using perpendicular magnetization films, the magnetic field straying from the reference layer acts on the storage layer, and therefore, the magnetic field needs to be cancelled. According to a suggested method to do so, a shifting magnetic-field correction layer is placed on the opposite side of the reference layer from the tunnel barrier layer. The magnetization direction of the shifting magnetic-field correction layer is antiparallel to the magnetization direction of the reference layer, and is adjusted so as to cancel the magnetic field acting on the storage layer.
In a variable-resistance memory using spin torque transfer switching writing, currents are applied to an MTJ element through the same path at the time of writing and at the time of reading. Therefore, there is a potential risk of “read disturb”, which is a phenomenon of data being rewritten at the time of reading. To avoid the risk, the read current to be applied to an MTJ at the time of reading is made smaller than the write current according to a method. By this technique, the read disturb probability is lowered. However, reducing the read current leads to a decrease in read sensitivity, and therefore, there is a limit to the reduction of practical read current.
By another suggested method to avoid read disturb, the read disturb probability is lowered by making the read current pulse width smaller than the write current pulse width. The read disturb probability can also be lowered by this method. However, in a memory required to perform high-speed operations, the write pulse width is made smaller in response to demand for high-speed writing. Therefore, read pulses need to be made even shorter. However, there is also a limit to the reduction of read pulse width, to avoid the problems with read sensitivity and pulse wiring delay.